The present invention relates to a servo control method of a disk drive mechanism. More particularly, the present invention relates to a method ofproviding for track follow of a read/write head at a quarter track position and a method of quarter track seeking using the track follow technique.
In a conventional disk drive system, the disk drive operates using two successive servo modes in accordance with the moving distance of a head. The first servo mode is concerned with a seek mode in which a head accesses tracks of a disk to search for and settle upon a desired target track. The second servo mode relates to a track following mode in which the head is accurately positioned on a data line of the target track for reading and writing data on the target position.
Conventional track follow methodology uses servo bursts that are typically designated as the xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d bursts. As shown in FIG. 1, the A, B, C and D bursts have varying amplitudes with respect to a track center line (TK center), such that when the head is positioned exactly over the center line of the target one of an odd or even track, approximately one-half of the amplitude of the A and B bursts will be read and when said head is positioned over the center line of the other of an odd or even track, approximately one-half of the amplitude of the C and D bursts will be read. As the head moves off the data line of the target track (ie, the center), the amplitude of one burst decreases while the amplitude of the other burst increases, depending on the direction of the misalignment. In this manner, a position error signal (PES) can be derived from the relative amplitudes of the A and B bursts, which are read in a timed displaced manner as the head passes over the bursts. Accordingly, ifthe head is properly centered with respect to the track, the difference of the A and B bursts (i.e., PES) equals zero.
An example of a conventional system within a disk drive to generate the PES is described in U.S. Pat. No. 4,415,939, entitled xe2x80x9cHead Positioning Servo for Disk Drivexe2x80x9d, issued on Nov. 15, 1983, which is incorporated herein by reference in its entirety. This system is illustrated in FIG. 2, which shows a control circuit 10 for controlling the position of the read/write head 12 with respect to the disk track. Window counter logic 30 is provided, which sequentially enables A and B detectors 32 and 34, respectively, upon initialization by the supply of the timing mark to the window counter logic 30. This input, together with the read clock signal from the data channel, indicates at what times the A and B bursts are expected to be present. The outputs of the detectors 32 and 34 are summed in a summing node 36. This A+B signal is compared with a constant value K in an automatic gain control loop to control the amplification provided by a voltage controlled amp 38 which amplifies the read back signal supplied to the detectors 32 and 34. This maintains a constant value of A+B independent of head flying height variations and the like, and allows the amplitudes of the A and B bursts to be compared reliably to one another for position error determination. The difference of the outputs of the detectors 32 and 34 is determined in a differential amplifier 40, thus providing the Axe2x88x92B signal which is then stored in a sample and hold circuit 42. As noted above, when the head is following the center of the disk track, Axe2x88x92B is zero. Hence, the position error signal is zero. It is passed to a comparator and compensator 44, where it is compared with a command signal. If the command signal is also zero, the output of the comparator and compensator 44 to the power amplifier 46 is zero. The output of the power amplifier 46 is supplied to the servo motor 18, which moves the servo arm 14, which carries the read/write head 12, which in turn supplies new servo position information to the voltage-controlled amplifier 38.
By applying a non-zero DC level to the command input of FIG. 2, the servo system may locate the read/write head at a position offset from track center. With the proper DC level applied to the command input, the read/write head may be moved to the quarter track position. However, as the head width becomes much narrower than the track width, the usable offtrack range is reduced. Referring again to FIG. 1, when the heads are moved beyond the quarter track position, there is a point at which the A and B signals reach a maximum and/or minimum level and then do not change further as the heads are moved further offtrack. At this point, the servo system becomes unstable, since Axe2x88x92B is constant, regardless of changes in track position. The quarter track position margin xe2x80x9cMxe2x80x9d as indicated in FIG. 1, is herein defined as the additional position offset which may be applied beyond quarter track before the servo system becomes unstable. FIG. 1 illustrates that as the head width becomes narrower with respect to the track width, the position margin M is reduced. If the head width approaches xc2xd the track width, the position margin approaches zero and the system becomes unstable. Also note that as the head width changes, the slope of the A and B amplitudes versus track position also changes. The amplitude of PES (Axe2x88x92B) versus track position is dependent on the head width. Likewise, the amount of DC offset required at the command input of FIG. 2 to locate the heads at quarter track will vary depending on the head width. Thus, if a constant pre-programmed voltage is used to command the quarter track offset, drive to drive variations in head width will create a DC positioning error about the quarter track point for the read/write head.
In view of the above, it can be recognized that conventional servo systems have a number of drawbacks when performing track follow at the quarter track position. As such, a need exists for providing a disk drive that implements a technique of utilizing servo patters that overcomes the drawbacks of the prior art.
In accordance with an aspect of the invention, a method of performing track follow at quarter track positions is provided that comprises reading servo bursts designated as A, B, C and D servo bursts located on the disk; deriving a position error signal from relative amplitudes of the servo bursts; and positioning the read/write head over the quarter track position in accordance with the position error signal. The method may include reading the servo bursts in a time-displaced manner, and the position error signal may be determined from the relative amplitudes of the servo bursts. The position error signal may be based on one of Cxe2x88x92A and Dxe2x88x92B when track following at xe2x88x92xc2xc track positions and one of Axe2x88x92D and Bxe2x88x92C when track following at +xc2xc track positions
In accordance with another aspect of the invention, there is provided a servo controller for controlling a position of a read/write head with respect to a track on a disk within a disk drive wherein the disk preferably includes servo bursts being designated as A, B, C and D servo bursts having varying amplitudes with respect to a center line of the track. The servo controller may comprise a digital control/timing circuit that controls timing of events within the servo controller and generates an offset signal; a servo demodulator that receives a read back signal from the read/write head and determines a difference signal; a summing node that receives the difference signal and the offset signal; and a programmable gain stage that generates a position error signal based on an output of the summing node.
According to a feature of the invention, the servo demodulator may receive the read back signal and, in accordance with a track position, demodulate selected servo bursts to determine a difference signal. The difference signal for even tracks may be determined as Cxe2x88x92A for xe2x88x92xc2xc track positions and Axe2x88x92D for +xc2xc track positions, and the difference signal for odd tracks may be determined as Dxe2x88x92B for xe2x88x92xc2xc track positions and Bxe2x88x92C for +xc2xc track positions. In addition, the offset signal is preferably ramped up and down during quarter track seek operations to minimize fluctuations of the position error signal. The programmable gain stage preferably maintains a constant loop gain for differing servo field difference signals.
According to yet another aspect of the invention, there is provided a method of performing quarter track seek operations in a disk drive having a servo control unit for controlling a position of a read/write head with respect to a track. The disk includes servo bursts having varying amplitudes with respect to a center line of the track, and the servo bursts are preferably designated as A, B, C and D servo bursts. The servo control unit may include a control/timing circuit that generates an offset signal, a demodulator that receives a read back signal from the read/write head and determines a difference signal, a summing node that receives the difference signal and the offset signal, a programmable gain stage that generates a position error signal based on an output of the summing node. The method of performing quarter track seek operations preferably comprises adjusting the offset signal a first amount; switching the difference signal from Axe2x88x92B to alternate servo fields; changing the loop gain; adjusting the offset signal a second amount; and allowing the read/write head to settle.
According to a feature of the invention, the step of adjusting the offset signal a first and second amount may comprise raising or lowering the offset signal 12.5%. Additionally, the step of switching the difference signal from Axe2x88x92B to the alternate servo fields preferably occurs at approximately xe2x85x9 of a track offset. Preferably, the alternate servo fields comprise one of Axe2x88x92D, Dxe2x88x92B, Cxe2x88x92A and Bxe2x88x92C.